Circuit / components required to detect nearby communication over a certain frequency range

Thread Starter


Joined May 28, 2013
Where I live, car theft is rampant. Brand new Mercs, Audis, BMWs and more being stolen from people's driveways. The methodology employed is a relay attack where a high gain amplifier of some sort is used to probe for /communicate with the remote fob. More detail below.

I wish to create a small device which buzzes when communication over the frequencies 433.92MHz (in Europe) or 315MHz (US) is detected. Let's say within a range of 2-3m (we don't want it detecting a signal 100m away!). Ideally a little bit more of the spectrum either side of the above frequencies should be detectable too. The idea would be to place the box somewhere near where the keyfob is stored. If during the night a thief attempts to detect the remote fob, a buzzer should go off and give the owner a chance to disrupt the attempted theft.

I have an emerging amateur interest in electronics and understand basic electrical concepts. I have played around with pots, digital/analog I/O boards, Arduino (have good coding skills in general). It's a start right?

I'm tying to understand some of the components required to make my idea happen and perhaps some suggestions on how to make it more reliable/fool proof. If anyone wants to shoot the idea down, please do so with a little explanation as to why it wouldn't work.

Many thanks



Joined Jan 8, 2017
How can you be sure that the thieves will use 433Mhz or 315Mhz for their link. If they are stealing cars I'm sure breaking licensing regulations will not bother them. They may also be able to use an infra red link which would not be detectable. I think the best solution is to keep your car keyfob in a screened box.



Joined Aug 21, 2008
If you find out what frequency range and the signaling protocol used for a particular car this might be a fairly straight-forward task, but somehow the detector in question would have to read the content of the communications between the fob and the car (through the relay), because the frequencies used for keyless entry are shared with many other uses.

It would not be good, for example, if your alarm went off because somebody turned on a Bluetooth speaker system.

For that reason, merely detecting signals within a certain frequency range might result in false alarms.

An alternative, though less inventive solution might be to keep the keyfob in a Faraday cage of some sort.

Edit: A simple solution with a low chance of false alarms might be to monitor the keyfob and sound the alarm when it answers the querying signal. That would be fairly easy to detect.
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